This may be a little bit way out there.
Some good answers are nuanced and may contain several options. Some of which are good or bad. Say there were three alternatives given in an answer, one brilliant and the other two rubbish. It would be nice to highlight the brilliant bit and upvote that specifically.
I think that the current way to do this is to put a comment explaining why you voted on a particular answer, but it may be nice to split the best bits out.
This does lean toward the SO as a wiki idea, where the best answer could float to the top but it could also end up a complete unintelligible mess.
Another way around this which is perhaps more practical is to encourage single answer answers. If people can think of a few alternatives to post them as separate answers and let the community decide.